Microporous membranes are useful as separators for primary and secondary batteries. Such batteries include lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, etc. When a microporous polyolefin membrane is used as a separator for a lithium ion battery, the membrane's properties significantly affect the battery's performance. In particular, the microporous polyolefin membrane's permeability and shutdown temperature generally affect the battery's performance.
It is desirable for a battery separator film to have a relatively low shutdown temperature compared to its meltdown temperature in order to improve battery safety margin. In general, microporous polyolefin membranes containing polyethylene have a relatively high shutdown temperature of about 132° C. to 140° C.
Microporous membranes having high strength and high permeability have been made using an ultra-high-molecular-weight polyolefin (hereinafter called “UHMWPO”). For example, JP60-242035A discloses a UHMWPO membrane made by a process comprising molding step of a gel-like sheet made by extruding a solution containing a UHMWPO having an average molecular weight ≧7×105 and a solvent, removing step of the solvent from the gel-like sheet, and then stretching step of the gel-like sheet. JP03-064334A discloses a polyolefin microporous membrane having a specific molecular weight distribution in order to produce the microporous membrane from a high-concentration UHMWPO solution.
Japanese patent applications JP60-023954A, JP03-201360A, and JP05-025305A disclose battery separator film comprising branched low density polyethylene (LDPE) and/or linear low density polyethylene (LLDPE). The film is said to have a relatively low shutdown temperature. Further, Japanese patent application JP11-269289A discloses a microporous membrane comprising 2 to 80 wt % of a substantially linear ethylene-alpha-olefin copolymer having a melting peak of 95° C. to 125° C. in order to reduce shutdown temperature. Though shutdown temperature is improved, the use of relatively low melting peak polyethylene can result in lower air permeability, particularly when film production, battery production, or battery use involves temperatures ≧the polyethylene's melting peak. Japanese patent application JP2002-338730A discloses that this loss of permeability can be partially overcome by using a polyethylene having a relatively higher melting peak, e.g., in the range of 125° C. to 132° C.